Duct burners for elevating the temperature of a stream of gas flowing through a conduit or the like operate by injecting and combusting fuel directly within the gas stream, with the combustion products comingling with the gas downstream of the burner. Where the gas stream being heated contains sufficient oxidant, the fuel is simply mixed with a portion of the flowing gas and the mixture ignited downstream of the fuel distribution system.
In order to operate both efficiently and reliably, the fuel distribution system of a duct burner must achieve the proper fuel-oxidant ratio, at least locally, over the entire burner operating range. It is also desirable to minimize the pressure drop and disruption of the gas stream passing through the duct burner arrangement in order to avoid flow losses and other inefficiencies which may result therefrom.
Prior art duct burning systems are typically designed to match the particular operating parameters of an individual application, for example temperature, gas flow velocity, fuel type, load range, etc. One particularly demanding application is in the use of a duct burner as a part of a thrust augmentor for a high performance aviation gas turbine engine. Such use, common in military and supersonic aircraft, requires a dependable, easily serviceable arrangement which is able to function with relatively high temperature gas streams and over a turndown ratio of up to 10:1 or greater.
One such prior art system, disclosed in U.S. Pat. No. 3,698,186 issued Oct. 17, 1972 to Beane et al shows a plurality of radial fuel spraybars distributed over the gas flow area of a duct burner or thrust augmentor for a gas turbine engine. The individual spraybars are divided into multiple segments corresponding to coaxial fuel distribution zones within the burner. Beane also discloses providing spraybars of differing length in an individual duct burner, resulting in a greater number of spraybar structures disposed in the outermost coaxial gas flow zone and progressively fewer spraybars in the intermediate and innermost zones.
Such prior art fuel distribution systems as are shown in Beane have a number of drawbacks which tend to reduce their efficiency and operability, particularly in high temperature, high performance thrust augmentor configurations. The use of differing length spraybars in an individual duct burner creates a non-uniform, discontinuous flow blockage distribution with respect to radial displacement, forcing a portion of the gas flowing adjacent the conduit or augmentor walls to flow radially inward in response to the greater fraction of the flow area obstructed by the spraybars. Such radial flow results in a nonuniformity of the radial velocity distribution downstream of the duct burner, reducing augmentor efficiency and thrust output.
Additionally, the termination of the shorter individual spraybars at differing radial displacements within the gas flow area initiates turbulent disruptions in the gas flow at the tip of each spraybar. Such disruptions, including for example trailing vortices extending downstream of the terminating tip of a shortened spraybar, disrupt the carefully optimized fuel-gas mixture created downstream of the fuel distribution system by the aerodynamically configured spraybars. For certain high temperature applications wherein the fuel gas mixture is close to its self-ignition temperature, the presence of even small flow disruptions caused by a terminating spraybar within the flowing gas stream can result in premature ignition of the fuel-gas mixture and thereby damage augmentor structures such as the fuel distributor, flameholder, etc.
What is needed is a fuel distribution and flameholding arrangement which avoids inducing radial flow within the gas stream, avoids inducing undesirable turbulence within the gas stream, and which provides the desired fuel-gas mixture ratio over a wide range of burner fuel and gas mass flow rates.